This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-401550 filed on Dec. 28, 2000.
The present invention relates to a method for producing silicon carbide (SiC) single crystal that is utilized for materials constituting a semiconductor device, a light-emitting diode or the like and to a producing apparatus therefor. Additionally, the invention relates to a SiC growth substrate for growing an SiC single crystal and to a method for heating the SiC single crystal.
The sublimation-recrystallization method has been widely used as a method for growing the SiC single crystal. In the proposed sublimation-recrystallization method, a seed crystal is supportably fixed to a graphite base disposed inside a graphite crucible, an SiC source material provided in the bottom of the crucible is heated and sublimated, a sublimated gas thereof is fed to the seed crystal, and thereby the SiC single crystal is grown on the seed crystal.
In the production of the SiC single crystal using the proposed sublimation-recrystallization method, a large defect (macroscopic defect) is generated from the interface between the seed crystal and the base. When a void exists between the seed crystal and the base when the seed crystal is fixed to the base, mass transfer from the seed crystal to the void is generated by sublimation and recrystallization inside the void. Thereby, the macroscopic defect is generated. Although a long SiC single crystal is obtained, due to the existence of the macroscopic defect, not only is it difficult to provide a large number of usable SiC wafers, but a hollow-shaped defect penetrating the crystal, so-called a micropipe, might also be induced by the macroscopic defect as a trigger. Thus, it is extremely difficult to obtain a high-quality SiC single crystal across a larger area by the proposed sublimation-recrystallization method.
To solve the above problem, in Japanese Patent Application Laid-Open No. 268096/1997, a seed crystal used in the sublimation-recrystallization method has a back surface, which is a back side of a plane where a single crystal is grown, covered with a stable protection layer. On the other hand, in Japanese Patent Application Laid-Open No. 110584/1997, a carbide layer interposing between a seed crystal and a base is applied to provide a homogeneous temperature distribution across a plane of the seed crystal.
However, as shown in FIG. 9A, when a seed crystal 3 is disposed inside a crucible 1 filled with a silicon carbide source material powder 2 and the surface of a protection layer 5 is fixed to the center of the ceiling of the crucible 1, which is called a base hereinafter, with an adhesive 6, a local temperature distribution is generated in the fixed plane because of thickness heterogeneity of the adhesive 6 or uneven contact of the protection layer 5 to the base 1b. Consequently, the protection layer 5 is damaged, or the protective function is diminished. Thereby, macroscopic defects 16 reaching deeply the inside of a silicon carbide single crystal 4 are generated.
The present invention has been made in view of the above-mentioned aspects with an object to prevent the macroscopic defect and to enable to produce a high-quality, long SiC single crystal.
The inventors found that if the seed crystal is supported mechanically without using an adhesive while growing so as not to damage the protection layer on the back surface of the seed crystal, or alternatively, if the seed crystal is supported only by the periphery thereof with an adhesive, a macroscopic defect are prevented.
According to a first feature of the invention, a protection layer is provided on the back surface of a seed crystal and the seed crystal is mechanically supported. Thereby, a local stress concentration due to the heterogeneous temperature distribution caused by uneven attachment of the seed crystal with an adhesive is not generated in the protection layer. Thus, the protection layer is not damaged, and a macroscopic defect is prevented.
This single crystal growing method is accomplished with many variations. For example, the seed crystal having the protection layer on the back surface is suspended and supported at the periphery thereof while an SiC single crystal is grown. Alternatively, the seed crystal is suspended and supported by a plurality of hook-shaped members while an SiC single crystal is grown. It is further preferable to use a seed crystal in a hexagonal shape in which the direction defined by a vertex and a diagonal vertex is set to be  less than 11{overscore (2)}0 greater than  and plane direction of the crystal is (0001), and to suspend and support the seed crystal by a plurality of hook-shaped members at the vicinity of vertices of the hexagon. For example, if the seed crystal is supported at three positions at the vicinity of the vertices, the generation of defects is prevented except at the three positions having the fastest crystal growth rate in expanding direction, and circular SiC wafers are efficiently cut out of a grown crystal.
Furthermore, it is preferable to support the SiC seed crystal with a predetermined gap between the protection layer provided on the back surface of the SiC seed crystal and a wall of a container facing thereto. By providing the predetermined gap, a local stress concentration due to the heterogeneous temperature distribution is not generated in the protection layer. Thus, the protection layer is not damaged and the macroscopic defect is prevented.
Preferably, a lid-shaped member facing to the protection layer on the SiC seed crystal is disposed for suppressing temperature distribution. A material making up the lid-shaped member has a thermal conductivity different from that of a seed crystal supporting member having a seed crystal supporting part. Thereby, the temperature distribution of the seed crystal is suppressed. By using a lid-shaped member having a thermal conductivity greater than that of the seed crystal supporting member, the temperature distribution of the crystal is readily suppressed. Thereby, a high-quality SiC single crystal with fewer macroscopic defects is obtained.
Besides, the SiC seed crystal may be supported in such a manner that the seed crystal closes an opening disposed in the wall of the container and the back surface thereof, where the protection layer is provided, is exposed to the outside space.
Applicable materials for the protection layer on the back surface of the SiC seed crystal are, specifically, a carbon layer, metal carbide layer with high melting point, silicon carbide epitaxial layer, silicon carbide polycrystalline layer, silicon carbide amorphous layer or multilayer film constituted of above layers. When a carbon layer, metal carbide layer with high melting point such as TaC, WC, MoC or TiC, silicon carbide epitaxial layer, or high density polycrystalline layer is formed on the back surface of the SiC seed crystal, the back surface of the SiC seed crystal is preferably protected. Thus, the protection layer escapes being damaged while growing and the generation of a macroscopic defect is suppressed.
Further preferably, a growth surface of the SiC seed crystal substrate, on which SiC crystal grows, is projected downwardly to the SiC source material side farther than a supported face, by which the SiC seed crystal substrate is supported. Thereby, the SiC single crystal grows without being unified with the supporting member.
These method and apparatus for producing a single crystal, substrate structure for growing a single crystal and method for heating a single crystal are also effective for other crystal growths, not limiting to SiC.